Page:VCH Kent 1.djvu/56

 A HISTORY OF KENT With the great white cliffs of this remarkable formation fronting the Channel at the nearest point of approach of our shores to the continent, in full view of all those who pass through this gateway of our seas, and with the bold scarp of the North Downs, which marks its prolongation into the interior no less conspicuous to the pilgrim by land, no other feature could indeed be more impressively characteristic of the county. The Downs form the highest ground in Kent, reaching elevations of between 700 and 800 feet in its western part, and 600 to 700 feet farther eastward. These heights are attained close to the steep escarp- ment in which the Chalk terminates, the surface declining thence gradually northward. It would be superfluous to describe the general aspect of the forma- tion, but we may dwell for a moment on the remarkably homogeneous composition of the Chalk, which is one of its most extraordinary features. Throughout the whole of its extent in England, from its lowest to its highest beds, with a thickness in some places reaching from 1,000 to 1,500 feet, and from its first appearance in the cliffs of the English Channel to its disappearance in Flamborough Head and the Yorkshire Wolds, this peculiar white limestone preserves everywhere its identity of character, with such minor modifications of structure as become apparent only when the formation is closely studied. It is one of the common- place ' wonders ' of geology that this huge mass has been built up almost entirely from the remains of lime-secreting organisms, among which the minute shells of foraminifera are especially abundant. For a period of time which is admitted to have been long even by geological measure- ment, and by any standard of human history would be reckoned inter- minable, the calcareous ooze derived from generation after generation of these organisms slowly accumulated on the floor of an open sea, too far from the coast to receive more than an inconsiderable sprinkling of current-borne detritus, and that usually of the lightest. At rare intervals however stones rafted from the land, perhaps by floating ice or entangled in the roots of seaweed or of fallen trees, were dropped to the sea bottom; and are occasionally found in the Chalk, as for example in the neighbour- hood of Gravesend,^ but their occurrence is quite exceptional. Nodules of flint, often occurring abundantly in bands or in tabular masses, are characteristic of a large part of the Chalk and form an integral portion of the deposit. Like the cherts of the Lower Greensand, their material has been mainly derived from the siliceous spicules of sponges, which are known to have flourished in large numbers in the seas of the period. Subdivisions of the Chalk. — The subdivision of this great mass into Lower, Middle, and Upper Chalk was originally based mainly upon slight differences of composition — the Lower Chalk being usually some- what grey in colour, marly in its lower portion, and devoid of flints ; the Middle division, white and rather flinty in places, sometimes with a hard rock-band (the 'Chalk Rock') at the top ; and the Upper Chalk, ' Mem. Geo!. Survey, 'Geology of London,' i. 82. 14